Currently yeasts are the organism or choice for the fermentative production of ethanol. Most common is the use of Saccharomyces cerevisiae, in processes using hexoses obtained from grains or mash as the carbohydrate source. Use of hydrolysate prepared from cellulosic biomass as a carbohydrate source for fermentation is desirable, as this is a readily renewable resource that does not compete with the food supply.
After glucose, the second most abundant sugar in cellulosic biomass is xylose, a pentose. Saccharomyces cerevisiae is not naturally capable of metabolizing xylose, but can be engineered to metabolize xylose with expression of xylose isomerase activity to convert xylose to xylulose, and additional pathway engineering.
Success in expressing heterologous bacterial xylose isomerase enzymes that are active in yeast has been limited. Some specific xylose bacterial isomerase sequences have been reported to provide xylose isomerase activity for a xylose utilization pathway in yeast. For example as U.S. Pat. No. 7,622,284 discloses a yeast cell expressing a xylose isomerase from Piromyces sp. US 2012/0184020 discloses eukaryotic cells expressing a xylose isomerase isolated from Ruminococcus flavefaciens. Similarly WO2011078262 disclose several xylose isomerases from each of Reticulitermes speratus and Mastotermes darwiniensis and proteins with high sequence identities to these, and their expression in eukaryotic cells. WO212009272 discloses constructs and fungal cells containing a xylose isomerase from Abiotrophia defectiva and others with sequence identity to it.
There remains a need for additional engineered yeast cells that express xylose isomerase activity for successful utilization of xylose, thereby allowing effective use of sugars obtained from cellulosic biomass during fermentation.